Many structures are comprised of plate-like features. In such structures, elastic waves may include Lamb waves as at least one form of elastic wave propagation therein. In addition, the structural material may be anisotropic in terms of its elastic properties, which is quite typical of structural composites. The result may be strong dispersion characteristics, causing a pulsed elastic waveform transmitted from an actuator to change shape and length significantly over the distance of the propagation path. The pulse may break up into several different modes, each of which may have different propagation velocities, depending on the wavelength relative to plate thickness, direction of propagation, and other factors. As a result, the received signal waveform may be very different and more complex as compared to the original pulse launched at the actuator. Specifically, the transmitted pulse may then include the first arrivals of different modes and scatters of these modes. However, because of the complexity of structures and the dispersion, these signal components may overlap and interfere with each other, making it very difficult to understand the composition of the signal received at a sensor of the SHM system.
The “first arrival” signal is the elastic wave component that propagates from an actuator to the sensor, and may be defined as the signal arriving at a time at which the signal exceeds a particular amplitude threshold. Since the property of this component is relatively easy to understand, it has been considered the most useful component for Lamb-wave-based structural health monitoring (SHM). The accuracy of first arrival detection is thus very important. The first arrival signal, however, may include a direct path propagation between the actuator and the sensor transducers plus a scattered, or reflected, wave from a damage in the neighborhood of the direct path. While the direct path propagating wave may not detect the damage, the scattered wave, when detected at the sensor transducer, will coherently interfere with the direct path wave, producing a more complicated received signal, which nevertheless, when properly treated, as described below, is useful in detection of damage.
Accurate detection of first arrival, however, may not be an easy task. Perhaps the most commonly used method now may be visual selection by inspection of the waveform data. However, this method is practically infeasible for the monitoring of relatively large structures, where thousands of signal paths may be implemented. Thus, automation of the process would be highly advantageous. The most straightforward automatic way is directly picking up the first signal wave packet that has relatively large components. However, this approach may fail in many situations because the electromagnetic interference (EMI) cross-talk between the actuation and sensor channels may easily exceed the magnitude of the electromechanically detected elastic wave, so that signal thresholding is not useful until cross-talk can be properly treated in an appropriate manner. The noisy shape of the elastic wave may increase the difficulty of automatically detecting the starting and ending points of the first signal arrival. Therefore, it is desirable to have methods and systems for the automatic removal of the EMI cross-talk, as well as the detecting of the first arrival signals.